Callahan, Cosgrove and maybe Childress too had been together as young coaches at Illinois in the 80's for Mike White. Then Alvarez got them all together at Wisconsin when he got that job in 90. Dan MCarney was BA's original DC, then when he went Iowa St, Cosgrove took over the D. Callahan was a heck of an OL coach.
A very good friend, (whom I didn't know at the time) was a HS AA DL recruit in the 80's. Said he really liked Callahan, who like my friend, hailed from the south side of Chicago, and wanted him bad in Champaign. But said the overall staff at Illinois just seemed too shady. He signed with ND/Holtz after also visiting UM and Minnesota.